In a shallow dish whisk oil and 1 tablespoon of lemon juice with pepper and garlic and add turkey. Marinate turkey, turning once, 30 minutes. Discard marinade. Heat a well-seasoned ridged grill pan over moderately high heat until hot and grill turkey 3 minutes on each side, or until just cooked through. Cool turkey on a cutting board and cut into 4 portions.

In a small food processor or blender puree watercress leaves and mayonnaise until smooth and blend in remaining tablespoon lemon juice and salt and pepper to taste.

Spread watercress mayonnaise on 8 toast slices and top 4 with turkey and salt and pepper to taste. Top turkey with 4 remaining mayonnaise-spread toasts, mayonnaise sides up, and top each portion with bacon, tomatoes, and watercress sprigs. Top with remaining toasts.